My sole .416 mould is the RCBS 416-350. In WW, this casts a 365-grain boolit at about .4165 diameter, and it works rather nicely in my Ruger #1.
I'm just starting a project to TRY finding a reliably-expanding cast-bullet load for a hunt this fall, and, deciding to start at the worst-possible end of the spectrum, I cast up some pure-lead boolits. These weighed-in at 369.8 grains, average of ten bullets.
My goal is to find a decent-shooting load at 2000 fps, since there's so much bullet weight available to supply the horsepower, and 2000 will offer a reasonable trajectory to 200 yards. Therefore, I selected a 52.0-grain charge of 5744 with dacron filler, which has given me just over 2000 fps in many repetitions with good accuracy.
Yesterday I hied meself forth to a gravel pit and set up Der Schuetzenwagen at 100 yards. A control group of five was fired with WW bullets, and yielded a 1.5" group, warming up the barrel and conditioning the bore as well (I hoped).
I then fired twenty rounds of pure-lead loads. There was NO visible leading, streaking, or other pollution of the bore! It remained as shiny-bright as it had been after the WW loads. The accuracy was noticeably poorer than the WW bullets delivered, but the 5-rounders were still in the 2.5"-3.5" area and all holes were nice and round. This accuracy level gives me considerable confidence, because I'm not all that attached to the pure-lead idea. I'm now going to move to a 50/50 mix of WW/lead, and see what I can get for groups with that alloy. I expect it'll do all right. If so, I'll try differential annealing as mentioned here recently, and soften-up the noses as much as is practical.
Since un-hardened WW alloy has worked well on game animals for others here, I expect that the softer metal may do even better. I'd like to have the bullet hang together without losing much weight while still expanding to a large degree, and this just might be the way to do it.
BTW, I fired five rounds yesterday with paper-patched 416-350s in pure lead
Grouping was about the same as unpatched bullets at 3.4", but I found that the patch increased the neck diameter of the cartridge enough that it took a VERY firm push to seat the round in the chamber. I had to use the handle end of a 3/8" breaker bar as a pusher to seat it far enough to raise the breechblock. Not exactly a great routine for the hunting field....